We’ve changed the app from single window to multiple window, which causes some “side effects” as well. The app now follows the standard multi window behavior of macOS apps. It means if you Close a window (red button) it will not stay in the background, as it used to do, because you can have other windows.

In the current version you can press the Hide Window button on the toolbar (crossed out eye). You can also minimize the window with yellow (-) button on the top left, or with cmd+M. You can also hide the app from DeskApp for YouTube menu.